Systems and methods for detecting defects on a wafer

ABSTRACT

Systems and methods for detecting defects on a wafer are provided. One method includes generating output for a wafer by scanning the wafer with an inspection system using first and second optical states of the inspection system. The first and second optical states are defined by different values for at least one optical parameter of the inspection system. The method also includes generating first image data for the wafer using the output generated using the first optical state and second image data for the wafer using the output generated using the second optical state. In addition, the method includes combining the first image data and the second image data corresponding to substantially the same locations on the wafer thereby creating additional image data for the wafer. The method further includes detecting defects on the wafer using the additional image data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/359,476 entitled “Systems and Methods for Detecting Defects on aWafer,” filed Jan. 26, 2009, which is incorporated by reference as iffully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to systems and methods fordetecting defects on a wafer. Certain embodiments relate to a methodthat includes combining different image data for substantially the samelocations on the wafer generated using different optical states of aninspection system to create additional image data that is used to detectdefects on the wafer.

2. Description of the Related Art

The following description and examples are not admitted to be prior artby virtue of their inclusion in this section.

Fabricating semiconductor devices such as logic and memory devicestypically includes processing a substrate such as a semiconductor waferusing a large number of semiconductor fabrication processes to formvarious features and multiple levels of the semiconductor devices. Forexample, lithography is a semiconductor fabrication process thatinvolves transferring a pattern from a reticle to a resist arranged on asemiconductor water. Additional examples of semiconductor fabricationprocesses include, but are not limited to, chemical-mechanical polishing(CMP), etch, deposition, and on implantation. Multiple semiconductordevices may be fabricated in an arrangement on a single semiconductorwafer and then separated into individual semiconductor devices.

Inspection processes are used at various steps during a semiconductormanufacturing process to detect defects on wafers to promote higheryield in the manufacturing process and thus higher profits. Inspectionhas always been an important part of fabricating semiconductor devicessuch as ICs. However, as the dimensions of semiconductor devicesdecrease, inspection becomes even more important to the successfulmanufacture of acceptable semiconductor devices because smaller defectscan cause the devices to fail. For instance, as the dimensions ofsemiconductor devices decrease, detection of defects of decreasing sizehas become necessary since even relatively small defects may causeunwanted aberrations in the semiconductor devices.

One obvious way to improve the detection of relatively small defects isto increase the resolution of an optical inspection system. One way toincrease the resolution of an optical inspection system is to decreasethe wavelength at which the system can operate. As the wavelength ofinspection systems decreases, incoherent light sources are incapable ofproducing light with sufficient brightness. Accordingly, for inspectionsystems that are designed to operate at smaller wavelengths, a moresuitable light source is a laser light source that can generaterelatively bright light at relatively small wavelengths. However, laserlight sources generate coherent light. Such light is disadvantageous forinspection since coherent light can produce speckle in images of awafer. Since speckle is a source of noise in the images, thesignal-to-noise ratio (S/N) images generated by inspection systems willbe reduced by speckle. In addition, speckle noise in wafer inspectionsystems (e.g., laser-based inspection systems) is one of the mainlimitations of defect of interest (DOI) detection ability. As waferdesign rules continue to shrink, optical inspection systems preferablyhave shorter wavelengths and larger collection numerical apertures(NAs). Speckle noise consequently increases to a more dominant noisesource.

Many illumination systems have been developed for inspectionapplications that reduce the speckle of light from laser light sources.For example, popular approaches to reduce speckle noise currentlyinvolve reducing coherence of the illumination laser source bytransmitting light through an optical diffuser or vibrating opticalfiber. These approaches usually require increasing the illumination NAon the wafer and therefore are not effective for an outside-the-lens(OTL) oblique angle illumination architecture. Reduction of lasercoherence also limits the usage of Fourier filtering and degrades theS/N. Other approaches such as moving an aperture in the pupil plane havebeen applied to select a spatial sample of light in the pupil plane andthen average the image over a relatively large number of samples. Thisapproach will greatly reduce the resolution of the optical systemthereby decreasing the defect capture rate.

Some methods for defect detection utilize output generated by multipledetectors of an inspection system to detect defects on a wafer and/or toclassify defects detected on the wafer. Examples of such systems andmethods are illustrated in International Publication No. WO 99/67626 byRavid et al., which is incorporated by reference as if fully set forthherein. The systems and methods described in this publication aregenerally configured to separately detect defects in the electricalsignals produced by different detectors. In other words, the electricalsignals produced by each of the detectors are processed separately todetermine if each detector has detected a defect. At any time that adefect is detected in the electrical signals produced by one of thedetectors, the electrical signals produced by at least two of thedetectors are analyzed collectively to determine scattered lightattributes of the defect such as reflected light intensity, reflectedlight volume, reflected light linearity, and reflected light asymmetry.The defect is then classified (e.g., as a pattern defect or a particledefect) based on these attributes.

Although the methods and systems disclosed in the above-referencedpublication utilize scattered light attributes of defects determinedfrom electrical signals generated by more than one detector, the methodsand systems disclosed in this publication do not utilize electricalsignals generated by more than one detector in combination to detect thedefects. In addition, the methods and systems disclosed in thispublication do not use a combination of electrical signals generated bymore than one detector for any defect-related function other thanclassification.

Other currently available inspection systems are configured to inspect awafer with more than one detection channel, to detect defects on thewafer by separately processing the data acquired by each of thechannels, and to classify the defects by separately processing the dataacquired by each of the channels. The defects detected by each of theindividual channels may also be further processed separately, forexample, by generating different wafer maps, each illustrating thedefects detected by only one of the individual channels. The defectdetection results generated by more than one channel of such a systemmay then be combined using, for example, Venn addition of the individualwafer maps. Such inspection may also be performed using output acquiredin a single pass or multiple passes. For example, one previously usedmethod for defect detection includes performing two or more scans of awafer and determining the union of the lot results as the finalinspection result for the wafer. In such previously used methods,nuisance filtering and defect binning is based on the Venn ID results,AND/OR operation, from multiple scans.

Such previously used inspection methods, therefore, do not leverage theoutput generated by the inspection system at the pixel level, but rathercombine the results at the wafer map level as the final result. Defectsare detected by each pass independently based on their relative signal(magnitude) compared to the wafer level noise seen for each pass. Inaddition, nuisance filtering and binning in previously used methods maybe based on the AND/OR detection from multiple scans and thereafterseparation in each individual scan. As such, no cross-pass informationother than the AND/OR operation on detection is considered.

Accordingly, it would be advantageous to develop methods and systems fordetecting defects on a wafer that combine information from differentoptical states of an inspection system to increase the S/N of defects inimage data for the wafer used for defect detection while decreasingnoise (e.g., speckle noise) in the image data.

SUMMARY OF THE INVENTION

The following description of various embodiments of methods, computerreadable media, and systems is not to be construed in any way aslimiting the subject matter of the appended claims.

One embodiment relates to a method for detecting defects on a wafer. Themethod includes generating output for a wafer by scanning the wafer withan inspection system using first and second optical states of theinspection system. The first and second optical states are defined bydifferent values for at least one optical parameter of the inspectionsystem. The method also includes generating first image data for thewafer using the output generated using the first optical state andsecond image data for the wafer using the output generated using thesecond optical state. In addition, the method includes combining thefirst image data and the second image data corresponding tosubstantially the same locations on the wafer thereby creatingadditional image data for the wafer. The method further includesdetecting defects on the wafer using the additional image data.

In one embodiment, the different values include different angles ofillumination at which light is directed to the wafer during thescanning. In another embodiment, scanning the wafer with the inspectionsystem using the first and second optical states is performed withcoherent light. In an additional embodiment, the first and secondoptical states are defined by the same values for optical parameters ofthe inspection system used for collecting light from the wafer duringthe scanning. In a further embodiment, the different values includedifferent imaging modes, different polarization states, differentwavelengths, different pixel sizes, or some combination thereof. Inanother embodiment, the different values include different channels ofthe inspection system. In one such embodiment, generating the outputusing the first and second optical states is performed in parallel.

In one embodiment, generating the output is performed in one pass. Inone such embodiment, the method also includes generating additionaloutput for the wafer by scanning the wafer in a different pass with theinspection system using the first or second optical state of theinspection system, generating different image data for the wafer usingthe additional output generated in the different pass, combining thedifferent image data with the first image data if the different pass isperformed using the first optical state or the second image data if thedifferent pass is performed using the second optical state correspondingto substantially the same locations on the wafer thereby creatingfurther additional image data for the wafer, and detecting defects onthe wafer using the further additional image data.

In another embodiment, the method includes generating output for thewafer by scanning the wafer with a different inspection system,generating third image data for the wafer using the output generatedusing the different inspection system, combining the third image datawith the first or second image data corresponding to substantially thesame locations on the wafer thereby creating further additional imagedata for the wafer, and detecting defects on the wafer using the furtheradditional image data.

In one embodiment, the first and second image data includes differenceimage data. In another embodiment, combining the first image data andthe second image data includes performing image correlation on the firstimage data and the second image data corresponding to substantially thesame locations on the wafer. In an additional embodiment, combining thefirst image data and the second image data is performed at the pixellevel of the first and second image data. In a further embodiment,defect detection is not performed prior to the combining step.

In one embodiment, portions of the additional image data that correspondto the defects have greater signal-to-noise ratios than portions of thefirst and second image data that are combined to create the portions ofthe additional image data. In another embodiment, the additional imagedata has less noise than the first and second image data. In anadditional embodiment, the additional image data has less speckle noisethan the first and second image data. In a further embodiment, themethod includes detecting defects on the wafer using the first imagedata, detecting defects on the wafer using the second image data, andreporting the defects detected on the wafer as a combination of thedefects detected using any of the first image data, the second imagedata, and the additional image data.

In one embodiment, the method includes determining values for featuresof the defects using the additional image data. In another embodiment,the method includes determining values for features of the defects usingsome combination of the first image data, the second image data, and theadditional image data.

In one embodiment, detecting the defects includes identifying potentialdefects on the wafer using the additional image data and identifying thedefects by performing nuisance filtering of the potential defects usingpixel level information about the potential defects determined using thefirst image data, the second image data, the additional image data, orsome combination thereof. In another embodiment, detecting the defectsincludes identifying potential defects on the wafer using the additionalimage data and identifying the defects by performing nuisance filteringof the potential defects using values for features of the defectsdetermined using the first image data, the second image data, theadditional image data, or some combination thereof.

In one embodiment, the method includes binning the defects using pixellevel information about the defects determining using the first imagedata, the second image data, the additional image data, or somecombination thereof. In another embodiment, the method includes binningthe defects using values of features of the defects determined using thefirst image data, the second image data, the additional image data, orsome combination thereof.

Each of the steps of each of the embodiments of the method describedabove may be further performed as described herein. In addition, each ofthe embodiments of the method described above may include any otherstep(s) of any other method(s) described herein. Furthermore, each ofthe embodiments of the method described above may be performed by any ofthe systems described herein.

Another embodiment relates to a computer-readable medium that includesprogram instructions executable on a computer system for performing acomputer-implemented method for detecting defects on a wafer. The methodincludes acquiring output for a wafer generated by scanning the waferwith an inspection system using first and second optical states of theinspection system. The first and second optical states are defined bydifferent values for at least one optical parameter of the inspectionsystem. The method also includes generating first image data for thewafer using the output generated using the first optical state andsecond image data for the wafer using the output generated using thesecond optical state. In addition, the method includes combining thefirst image data and the second image data corresponding tosubstantially the same locations on the wafer thereby creatingadditional image data for the wafer. The method further includesdetecting defects on the wafer using the additional image data.

Each of the steps of the computer-implemented method described above maybe further performed as described herein. In addition, thecomputer-implemented method may include any other step(s) of any othermethod(s) described herein. The computer-readable medium may be furtherconfigured as described herein.

An additional embodiment relates to a system configured to detectdefects on a wafer. The system includes an inspection subsystemconfigured to generate output for a wafer by scanning the wafer usingfirst and second optical states of the inspection subsystem. The firstand second optical states are defined by different values for at leastone optical parameter of the inspection subsystem. The system alsoincludes a computer subsystem configured to generate first image datafor the wafer using the output generated using the first optical stateand second image data for the wafer using the output generated using thesecond optical state. The computer subsystem is also configured tocombine the first image data and the second image data corresponding tosubstantially the same locations on the wafer thereby creatingadditional image data for the wafer. In addition, the computer subsystemis configured to detect defects on the wafer using the additional imagedata. The system may be further configured as described herein.

A further embodiment relates to another method for detecting defects ona wafer. This method includes generating output for a wafer by scanningthe wafer with an inspection system in first and second passes using afirst optical state of the inspection system. The method also includesgenerating first image data for the wafer using the output generated inthe first pass and second image data for the wafer using the outputgenerated in the second pass. In addition, the method includes combiningthe first image data and the second image data corresponding tosubstantially the same locations on the wafer thereby creatingadditional image data for the wafer. The method further includesdetecting defects on the wafer using the additional image data.

Each of the steps of the method described above may be further performedas described herein. In addition, the method described above may includeany other step(s) of any other method(s) described herein. Furthermore,the method described above may be performed by any of the systemsdescribed herein.

Still another embodiment relates to another method for detecting defectson a wafer. This method includes generating output for a wafer byscanning the wafer with first and second inspection systems. The methodalso includes generating first image data for the wafer using the outputgenerated using the first inspection system and second image data forthe wafer using the output generated using the second inspection system.In addition, the method includes combining the first image data and thesecond image data corresponding to substantially the same locations onthe wafer thereby creating additional image data for the wafer. Themethod further includes detecting defects on the wafer using theadditional image data.

Each of the steps of the method described above may be further performedas described herein. In addition, the method described above may includeany other stem's) of any other method(s) described herein. Furthermore,the method described above may be performed by any of the systemsdescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention may become apparent to thoseskilled in the art with the benefit of the following detaileddescription of the preferred embodiments and upon reference to theaccompanying drawings in which:

FIG. 1 is a schematic diagram illustrating a side view of one embodimentof different optical states of an inspection system that are defined bydifferent values for at least one optical parameter of the inspectionsystem;

FIG. 2 includes different image data for the same location on a wafer,each of which was generated using output that was generated using one ofthe different optical states of FIG. 1;

FIG. 3 includes different output generated for the same location on awafer using the different optical states of FIG. 1;

FIG. 4 is image data generated using one example of the output of FIG.3;

FIG. 5 is additional image data created by combining the image data ofFIG. 4 with other image data generated using the other example of theoutput of FIG. 3;

FIG. 6 is a block diagram illustrating one embodiment of acomputer-readable medium that includes program instructions executableon a computer system for performing a computer-implemented method fordetecting defects on a wafer; and

FIG. 7 is a schematic diagram illustrating a side view of one embodimentof a system configured to detect defects on a wafer.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples of such asemiconductor or non-semiconductor material include, but are not limitedto, monocrystalline silicon, gallium arsenide, and indium phosphide.Such substrates may be commonly found and/or processed in semiconductorfabrication facilities.

One or more layers may be formed upon a wafer. Many different types ofsuch layers are known in the art, and the term wafer as used herein isintended to encompass a wafer on which all types of such layers may beformed. One or more layers formed on a wafer may be patterned. Forexample, a wafer may include a plurality of dies, each having repeatablepatterned features. Formation and processing of such layers of materialmay ultimately result in completed semiconductor devices. As such, awafer may include a substrate on which not all layers of a completesemiconductor device have been formed or a substrate on which all layersof a complete semiconductor device have been formed.

The wafer may further include at least a portion of an integratedcircuit (IC), a thin-film head die, a micro-electro-mechanical system(MEMS) device, fiat panel displays, magnetic heads, magnetic and opticalstorage media, other components that may include photonics andoptoelectronic devices such as lasers, waveguides and other passivecomponents processed on wafers, print heads, and bio-chip devicesprocessed on wafers.

Turning now to the drawings, it is noted that the figures are not drawnto scale. In particular, the scale of some of the elements of thefigures is greatly exaggerated to emphasize characteristics of theelements. It is also noted that the figures are not drawn to the samescale. Elements shown in more than one figure that may be similarlyconfigured have been indicated using the same reference numerals.

One embodiment relates to a method for detecting defects on a wafer. Themethod includes generating output for a wafer by scanning the wafer withan inspection system using first and second optical states of theinspection system. The output generated by scanning the wafer mayinclude any suitable output and may vary depending on the configurationof the inspection system and/or the inspection recipe used to performthe scanning. For example, the output may include signals, data, images,or image data responsive to light scattered from the wafer (e.g., in thecase of dark field (DF) inspection systems).

The inspection system may be a commercially available inspection systemsuch as the Puma 91xx series tools, which are commercially availablefrom KLA-Tencor, San Jose, Calif. The inspection system may beconfigured for inspection of patterned wafers and/or unpatterned wafers.In addition, the inspection system may be configured for DF inspection,possibly in combination with one or more other inspection modes (e.g.,an aperture mode of inspection). Furthermore, the inspection system maybe configured as an optical inspection system. Scanning the wafer withthe inspection system may be performed in any suitable manner. Forexample, the wafer may be moved (by a stage of the inspection system)with respect to optics of the inspection system such that theillumination of the inspection system traces a serpentine path over thewafer as light scattered from the wafer is detected.

The first and second optical states are defined by different values forat least one optical parameter of the inspection system. For example, anoptical “state” (which may also be commonly referred to as an optical“configuration” or “mode”) of the inspection system can be defined byvalues for different optical parameters of the inspection system thatare or can be used in combination to generate output for a wafer. Thedifferent optical parameters may include, for example, wavelength ofillumination, wavelength of collection/detection, polarization ofillumination, polarization of collection/detection, illumination angle(defined by elevation angle or angle of incidence and possibly azimuthalangle), angle of collection/detection, pixel size, and the like. Thefirst and second optical states may be defined by different values foronly one of the optical parameters of the inspection system and the samevalues for other optical parameters of the inspection system. However,the first and second optical states may be defined by different valuesfor two or more of the optical parameters of the inspection system.

In one embodiment, the different values include different angles ofillumination at which light is directed to the wafer during thescanning. The different angles of illumination may include substantiallythe same elevation angle and different azimuthal angles. FIG. 1illustrates one such embodiment of different optical states of aninspection system that are defined by different values for at least oneoptical parameter of the inspection system. For example, as shown inFIG. 1, light 10 may be directed to wafer 12 at azimuthal angle 14(e.g., an azimuthal angle of about 45 degrees). Light 16 may be directedto the wafer at azimuthal angle 18 (e.g., an azimuthal angle of about−45 degrees). Light 10 and light 16 may be directed to the wafer at thesame or substantially the same elevation angle 20 (e.g., about 15degrees). However, light 10 and light 16 may be directed to the wafer atdifferent elevation angles and/or different azimuthal angles. Light 10and light 16 may be generated by different light sources or the samelight source.

Light 10 and light 16 may have substantially the same characteristics(e.g., wavelength, polarization, etc.). In this manner, in order toseparately detect the light scattered from the wafer due to illuminationat the different illumination angles to thereby generate separate outputfor the different optical states, the wafer may be scanned with light atthe different illumination angles in different passes (i.e., multiplepasses performed in a single process). For example, in a double passinspection, first pass output may be generated with illumination comingin at a certain elevation angle and a 45 degree azimuthal angle. Secondpass output may be generated with the same optical conditions used forthe first pass except at −45 degree azimuthal angle illumination.

Such multiple pass (or multi-pass) output generation may be performedfor any different optical states of the inspection system for whichoutput cannot be simultaneously and separately generated (e.g., due to adifference in a setting of a single optical element of the inspectionsystem between the different optical states). However, if the differentoptical states of the inspection system can be used to simultaneouslygenerate separate output for the wafer using different channels of theinspection system), then generating the output using the first andsecond optical states of the inspection system may be performed in asingle pass scan of the wafer.

In another embodiment, scanning the wafer with the inspection systemusing the first and second optical states is performed with coherentlight. The coherent light may include light generated by any suitablecoherent light source (e.g., a laser) at any suitable wavelength. Inaddition, the method may be performed using an outside-the-lens (OTL)optical inspection system in which the illumination source is laserlight incident on the wafer at an oblique angle of incidence. In onesuch embodiment, as shown in FIG. 1, light 10 and light 16 may bedirected to the wafer at an oblique angle of incidence and outside oflens 22 of the inspection system. Lens 22 may be configured to collectlight scattered from the wafer due to illumination of the wafer duringscanning. An inspection system that includes lens 22 may be furtherconfigured as described herein.

In this manner, one advantage of the embodiments described herein isthat the embodiments can reduce speckle noise as described furtherherein; and compared to other common approaches for reducing specklenoise, the coherence of the laser source may be preserved. Therefore, inthe embodiments described herein, a Fourier filtering technique can beapplied effectively to eliminate pattern background in a DF geometry.The Fourier filtering technique may include any (optical or dataprocessing) Fourier filtering technique known in the art. Although theoutput may be advantageously generated in the embodiments describedherein by scanning the wafer using coherent light in an OIL illuminationconfiguration, the output may be generated using any suitable light inany suitable illumination configuration.

In an additional embodiment, the first and second optical states aredefined by the same values for optical parameters of the inspectionsystem used for collecting light from the wafer during the scanning. Forexample, as described above, the values may include different angles ofillumination at which light is directed to the wafer during scanning. Inaddition, the different optical states may be different only in one ormore illumination optical parameters of the inspection system. As such,no change in collection optical path between the different opticalstates (using which output may be generated in possibly different passesor scans of the wafer) may be made since only the illumination may bechanged. Using the same collection optical path for the differentoptical states may advantageously reduce the alignment error and opticalerror between different image data, which may be generated as describedherein using output acquired by scanning the wafer in two or morepasses) and which may be combined as described further herein.

In a further embodiment, the different values include different imagingmodes, different polarization states, different wavelengths, differentpixel sizes, or some combination thereof. For example, the differentvalues may include different polarization states for illumination. Inone such example, the first and second optical states may be defined bythe same polarization states for collection. For example, the differentvalues may include the p-polarized (P) state for illumination in oneoptical state and the s-polarized (S) state for illumination in theother optical state, and the polarization state used for collection inboth optical states may be unpolarized (N). However, in another suchexample, the optical states may also be defined by differentpolarization states for collection. For example, the first optical statemay be defined by the S polarization state for illumination and the Ppolarization state for collection, and the second optical state may bedefined by the P polarization state for illumination and the Spolarization state for collection.

In another embodiment, the different values include different channelsof the inspection system. For example, the first optical state may bedefined by a first channel of the inspection system, and the secondoptical state may be defined by a second channel of the inspectionsystem. In other words, the output for the wafer may be generated forthe first optical state using one channel of the inspection system, andthe output for the wafer may be generated for the second optical stateusing a different channel of the inspection system. The term “channel”is generally used herein to refer to different detection subsystems ordetectors of the inspection system, which may be different in angles(i.e., collection angles) at which light from the wafer is collected anddetected by the detection subsystems or detectors, but which may or maynot be different in other respects as well (e.g., wavelength(s) at whichlight is detected by the channels, polarization of the light detected bythe channels, etc.).

In one such example, if the inspection system includes three channels,the first and second optical states may be defined by the followingchannel combinations: channel 1 and channel 2; channel 2 and channel 3;and channel 1 and channel 3. In addition, as described further herein,the embodiments may be performed using more than two different opticalstates. In one such example, if the inspection system includes threechannels, first, second, and third optical states may be defined bychannel 1, channel 2, and channel 3, respectively. Furthermore, each ofthe different optical states may be defined by a different channel ofthe inspection system (e.g., N optical states defined by N channels).

In one such embodiment, generating the output using the first and secondoptical states is performed in parallel. For example, the outputgenerated using the first and second optical states may be generated inthe same pass or scan. As such, the output from each channel may becollected in parallel.

The method also includes generating first image data for the wafer usingthe output generated using the first optical state and second image datafor the wafer using the output generated using the second optical state.In one embodiment, the first and second image data includes differenceimage data. The difference image data may be generated in any suitablemanner. For example, difference image data for the first optical statemay be generated using test image data and two references (e.g., imagedata from dies on the wafer that are adjacent to the die from which thetest image data was generated). In such an example, one reference may besubtracted from the test image data, and the other reference may beseparately subtracted from the test image data. The results of bothsubtraction operations may be multiplied, and the absolute value of thatproduct may be the difference image data. Difference image data for thesecond optical state may be generated in a similar manner. As such, thedifference image data may be separately generated for each optical stateusing only output generated using that optical state. In other words,generating the difference image data is not a cross-optical stateoperation. In this manner, generating the first and second image datamay include performing a die-to-die subtraction to eliminate patternbackground in the output. However, the difference image data may begenerated in any other suitable manner using any suitable algorithm(s)and/or method(s). In addition, the first and second image data may notbe difference image data. For example, the first and second image datamay be raw image data of the wafer after any other pattern backgroundelimination operation(s) (e.g., after Fourier filtering).

The method also includes combining the first image data and the secondimage data corresponding to substantially the same locations on thewafer thereby creating additional image data for the wafer. In thismanner, the first and second image data may be combined on alocation-to-location basis. Unlike other methods that involve combininginformation about a wafer acquired using different optical states of aninspection system, combining the first image data and the second imagedata as described herein creates different image data for the wafer,which can then be used as described further herein (e.g., for defectdetection). For example, combining the first and second image data mayinclude performing “image fusion” using the first image data and thesecond image data. In other words, new image data of the wafer may be“fused” from two other image data of the wafer. As such, image fusionmay be performed using multiple optical states, which may include any ofthose described herein (e.g., optical states defined by differentpolarizations, different channels, etc.). For example, image fusion canbe achieved by using image data from any two (or more) channels of theinspection system, in one such example, if the inspection systemincludes three channels, image fusion can be performed using thefollowing channel combinations: channel 1 and channel 2; channel 2 andchannel 3; channel 1 and channel 3; and channels 1, 2, and 3. Inaddition, as described further herein, the output used to generate thefirst image data and the second image data may be acquired in differentpasses. In this manner, the method may include multi-pass image fusion.However, as also described further herein, the output used to generatethe first and second image data may be acquired in a single pass (e.g.,output from each channel may be collected in parallel). As such, themethod may include single-pass image fusion.

Although the method includes combining the first image data and thesecond image data as described above, the method is not limited tocombining only the first image data and the second image data. Forexample, if output is generated for the wafer using a third opticalstate of the inspection system, which is defined by at least one valuefor at least one optical parameter of the inspection system that isdifferent than the values for that at least one optical parameter whichdefine the first and second optical states, the method may includegenerating third image data for the wafer using the output generatedusing the third optical state, which may be performed as describedherein. The third optical state may be defined by any of the differentvalues for any of the optical parameters described herein. In one suchexample, each of the different optical states may be defined by adifferent channel of the inspection system. The method may also includecombining the first image data, the second image data, and the thirdimage data corresponding to substantially the same locations on thewafer as described herein thereby creating the additional image data forthe wafer. Combining image data generated using output generated by twoor more optical states is advantageous as described further herein.

Furthermore, although the method includes creating additional image datafor the wafer, the method is not limited to creating only the additionalimage data for the wafer. For example, the method may include creatingthe additional image data for the wafer as described above and creatingdifferent additional image data for the wafer in a similar manner. Inone such example, the additional image data may be created by combiningthe first and second image data as described above. The method may alsoinclude generating output for the wafer using a third optical state ofthe inspection system, which may be defined as described further herein.That output may be used to generate third image data for the wafer asdescribed further herein. That third image data may then be combined asdescribed herein with the first image data and/or the second image datato create different additional image data. The different additionalimage data may be used in a manner similar to using the additional imagedata in steps described further herein.

In one embodiment, combining the first image data and the second imagedata includes performing image correlation on the first image data andthe second image data corresponding to substantially the same locationson the wafer. For example, new wafer image data or the fused image datamay be generated by correlating image data (e.g., from two passes). Inone example, the image correlation may include a 5 pixel by 5 pixelcorrelation. However, the image correlation may be performed in anyother suitable manner using any suitable image correlation algorithm(s)and/or method(s). In addition, the image correlation may be performedusing any suitable image processing technique that can be used for imagecorrelation.

In another embodiment, combining the first image data and the secondimage data is performed at the pixel level of the first and second imagedata. In other words, the first and second image data may be combined ona pixel-by-pixel basis. In still other words, combining the first andsecond image data may be performed separately for individual pixels inthe first and second image data. By fusing information at the pixellevel, one can leverage both magnitude (intensity) and phase(correlation) information among different optical states (which may begenerated by different inspection passes). By combining information atthe pixel level, a new dimension to exploit becomes available, namelythe coincidence among different perspectives (optical states).

The first and second image data (e.g., difference image data) for thedifferent optical states may be generated for the entire wafer or theentire portion of the wafer that is scanned with the inspection systemusing the first and second optical states. In addition, combining thefirst and second image data may be performed using all of the first andsecond image data. In this manner, image fusion may be performed for theentire wafer or the entire portion of the wafer that is scanned usingthe first and second optical states.

However, image fusion may not be performed for the entire wafer or theentire scanned portion of the wafer. For example, the method may includeapplying an intensity cut off to the first and/or second image data andeliminating any of the first and/or second image data that does not haveintensity values that exceed the intensity cut off. In this manner, thefirst and/or second image data that is not eliminated may be identifiedas candidates to be used for additional steps performed in the method.In one such example, if the first image data is generated using outputgenerated in a first pass of the wafer and the second image data isgenerated using output generated in a second pass of the wafer, theintensity cut off may be applied to the first image data to eliminateany of the first image data that does not have intensity values thatexceed the intensity cut off. In this manner, the method may includesaving image patch data for only the candidates identified in the firstpass. In the second pass, only second image data that corresponds to thesame locations on the wafer as the candidates may be stored and/orcombined with the first image data. In this manner, the image data thatis saved in the second pass may vary depending on the candidates thatwere captured in the first pass, and image fusion may then be performedusing the image data saved in the second pass. However, if the first andsecond image data is generated using output that is generated in asingle pass of the wafer, the intensity cut off may be applied to boththe first and second image data and any of the first and second imagedata that has values that exceed the intensity cut off may be combinedwith the corresponding image data regardless of the intensity values ofthat other image data.

The method may include performing some (i.e., one or more) dilationsteps to ensure proper alignment between the defect signals in the imagedata. For example, for each of the candidates identified as describedabove, a 3 pixel by 3 pixel dilation may be performed. However, thedilation stem's) may include any suitable dilation image processingtechnique(s) known in the art and may be performed using any suitablemethod(s) and/or algorithm(s). The dilation step(s) may be performed onboth the first and second image data thereby increasing the accuracywith which defect signals in the first and second image data are alignedto each other.

Regardless of whether or not the method includes dilation step(s) suchas those described above, the method may include aligning the first andsecond image data prior to the combining step. Image data alignment maybe performed in any suitable manner. For example, image data alignmentmay be performed through cross-correlation of X and Y projections (e.g.,of the average intensity across the image data in the x and ydirections) between image data generated for different optical states(e.g., from image data acquired in two passes).

In an additional embodiment, defect detection is not performed prior tothe combining step. For example, as described above, an intensity cutoff may be applied to the first and/or second image data prior to thecombining step. However, the intensity cut off is not a defect detectionthreshold, method, or algorithm. Instead, the intensity cut off actsessentially as a noise filter to eliminate the image data that does nothave relatively high intensity values only for the purpose of decreasingthe processing involved in other steps of the method. In addition,defect detection may be performed as described further herein using thefirst and second image data individually, and defect detection using thefirst and/or second image data may or may not be performed prior toperforming the combining step. However, defect detection cannot beperformed using the additional image data until after the combining stepin which the additional image data is created has been performed. Inthis manner, unlike methods and systems that involve combininginformation generated after defect detection (e.g., combining defectdetection results from different scans of a wafer), the embodimentsdescribed herein combine information prior to defect detection, which isadvantageous as described further herein.

Since the embodiments described herein include combining first andsecond image data to create additional image data, a fairly substantialamount of image data may be stored during the method. Examples ofmethods and systems that are particularly suitable for storingrelatively large amounts of data such as image data are described incommonly owned U.S. patent application Ser. No. 12/234,201 by Bhaskar etal. filed Sep. 19, 2008, now U.S. Pat. No. 8,126,255 issued on Feb. 28,2012, which is incorporated by reference as if fully set forth herein.The embodiments described herein may include storing the output and/orimage data generated by the embodiments described herein using themethods and systems described in this patent application. For example, asystem may include eight image computers. During the first pass of amulti-pass inspection performed with a first optical state, each imagecomputer may receive and store ⅛ of the image data for each swathscanned on a wafer. During the second pass of the multi-pass inspectionperformed with a second optical state, each image computer may receiveand store ⅛ of the image data for each swath scanned on the wafer. Inaddition, each image computer may receive and store image data generatedat substantially the same locations on the wafer in both passes (i.e.,image data generated at substantially the same wafer locations and/orsubstantially the same in-swath positions). The image data generatedduring the second pass may be stored in the image buffers of the imagecomputers at fixed offset locations from the locations of the storedfirst pass image data. The stored image data may then be used in any ofthe step(s) described herein. The computer systems and computersubsystems described herein may be further configured as described inthe above-referenced patent application. The embodiments describedherein may also include any step(s) of any method(s) described in theabove-referenced patent application.

The method further includes detecting defects on the wafer using theadditional image data. Therefore, defect detection is no longer onlydetermined by each optical state (or each pass) independently, but basedon information fused from multiple optical states (e.g., all passes). Inthis manner, the methods described herein use image fusion results,which are generated by combining information from raw (difference) imagedata generated by multiple optical states, as the input to defectdetection. The defects detected on the wafer using the additional imagedata may include any defects known in the art and may vary depending onone or more characteristics of the wafer (e.g., the wafer type or theprocess performed on the wafer prior to inspection).

Detecting the defects using the additional image data may includeapplying one or more defect detection thresholds to the additional imagedata. For example, the additional image data may be compared to one ormore defect detection thresholds. The one or more defect detectionthresholds can be used to make a decision regarding whether a pixel inthe additional image data is defective or not. Other methods for defectdetection using one or more defect detection thresholds may first selecta set of candidate pixels using a simpler (less computationallyinvolved) test followed by a more complex computation applied only tothe candidates to detect defects.

One or more detect detection thresholds that are used to detect thedefects on the wafer may be defect detection threshold(s) of one or moredetect detection algorithms, which may be included in an inspectionrecipe. The one or more defect detection algorithms that are applied tothe additional image data may include any suitable defect detectionalgorithm(s) and may vary depending on, for example, the type ofinspection that is being performed on the wafer. Examples of suitabledefect detection algorithms, which can be applied to the additionalimage data, include segmented auto-thresholding (SAT) or multiple dieauto-thresholding (MDAT), which are used by commercially availableinspection systems such as those from KLA-Tencor. In this manner, theadditional image data may be treated as any other image data when itcomes to defect detection.

In one embodiment, the additional image data has less noise than thefirst and second image data. For example, combining the image data forthe wafer generated for different optical states as described hereinoffers new context of the noise profile of the wafer and sensitivity todefects of interest (DOI). In addition, by combining (or fusing)information from multiple optical states at the pixel level, thesensitivity to nuisance events or noise can be reduced. For example, byperforming the image correlation as described above, wafer noise in thefirst image data and the second image data that is non-spatiallycoincident can be substantially eliminated in the additional image data.In this manner, the embodiments described herein leverage the fact thatdifferent optical states (e.g., defined by different imaging modes,polarization states, wavelengths, pixel sizes, etc.) provide differentperspectives of the wafer level noise and nuisance defects therebyoffering the potential to suppress noise in the additional image data,which may be used as described further herein (e.g., for defectdetection purposes).

In an additional embodiment, the additional image data has less specklenoise than the first and second image data. For example, the embodimentsdescribed herein may use an image correlation process (to create theadditional image data) thereby substantially eliminating un-correlatedspeckle noise. In addition, as described above, the first and secondoptical states may be defined by different illumination angles. As such,the embodiments described herein may be used for speckle noisesuppression by varying illumination angle. In other words, theembodiments described herein may be used for speckle noise suppressionby correlating image data generated using output that is generated usingvarious illumination angles in an optical inspection system. Forexample, as the illumination angle changes, the phase relationship ofthe scattered light from the surface roughness on the wafer changes. Thespeckle pattern in the image data changes accordingly. When this changeis sufficient, a correlation of different image data will help toeliminate the speckle noise.

An example of how the speckle pattern changes as illumination anglechanges is shown in FIG. 2. inspection image data 24 and 26 wasgenerated for the same location on a wafer. Inspection image data 24 wasacquired at an illumination azimuthal angle of 45 degrees. Inspectionimage data 26 was acquired at an illumination azimuthal angle of −45degrees. Therefore, the inspection image data was acquired at differentillumination angles (e.g., the same elevation angle but differentazimuthal angles). Portions 28 of the image data show that the specklesignatures produced by the two azimuthal angle illuminations aresubstantially different at the same location on a page break. Inparticular, the bright speckle spot in inspection image data 26 iscaused by surface roughness and will contribute as noise or nuisance. Ininspection image data 24, the bright speckle spot is not present at thesame location as the speckle pattern changes with the illuminationangle. Therefore, the wafer noise changes as the azimuthal angle ofillumination changes. In particular, the bright speckle disappears asthe azimuthal angle is changed from −45 degrees to 45 degrees.

In one embodiment, portions of the additional image data that correspondto defects on the wafer have greater signal-to-noise ratios (S/Ns) thanportions of the first and second image data that are combined to createthe portions of the additional image data. For example, by combining (orfusing) information from multiple optical states at the pixel level,weak signal strengths from may be enhanced. Enhancing the signalstrengths from DOI may be achieved by not only taking advantage of therelative signals for defects in each optical state (magnitude), but byalso exploiting coincidence or correlations among the different opticalstates (phase). For example, fusing information at the pixel levelthereby leveraging both magnitude (intensity) and phase (correlation)information among different optical states allows one to extract defectswith weak signals and suppress noise and nuisance events by exploitingtheir respective coincidence and non-coincidence for different opticalstates. In this manner, the embodiments described herein leverage thefact that different optical states (e.g., defined by different imagingmodes, polarization states, wavelengths, pixel sizes, etc.) providedifferent perspectives of the wafer level noise and nuisance defectsthereby offering the potential to enhance the contrast of the DOI andtheir separation from nuisance defects. In addition, pixel level imagefusion across multiple optical states provides opportunities forenhancement of separation between DOI and nuisances although both mayhave relatively high S/Ns.

In one such example, when the change in the speckle pattern issufficient between different optical states used to generate the outputfor the wafer, a correlation of different image data for the differentoptical states will help to eliminate the speckle noise and improve theS/N as the signal scattering intensity from the defect may be relativelyconstant. For example, if the illumination angles are symmetric inoptical architecture, defect signals may be similar in both opticalstates, which is especially true for relatively small defects. Whilereducing speckle noise after image correlation, the defect signal ismaintained during the process. In this manner, the embodiments describedherein can maintain a healthy defect signal level after imagecorrelation. For example, compared to other common approaches forreducing speckle noise, the speckle noise is selectively eliminatedinstead of averaging over a relatively large sample of speckle patterns.Selectively eliminating the speckle noise instead of averaging over arelatively large sample of speckle patterns helps to reduce the noisefloor and improve the S/N.

In this manner, as the examples described herein illustrate, the defectS/N in the additional image data is greatly improved over the S/N of thedefect in each individual optical state, especially on wafers wherespeckle noise is dominant. For example, a defect that is not detectableusing either optical state (using either difference image data generatedusing one illumination angle) may become detectable after imagecorrelation. In particular, one advantage of the embodiments describedherein is that speckle noise can be greatly reduced in the additionalimage data compared to the first and second image data while defect S/Nsin the additional image data are improved compared to the first andsecond image data. As such, a defect that is not detectable in either ofthe first and second image data may become detectable in thecorresponding additional image data created by image correlation.

However, the embodiments described herein can be used to increase theS/Ns for defects that are detectable in either one or both of the firstand second image data individually (e.g., using image data from oneillumination angle and/or using image data from a different illuminationangle). For example, even if a defect produces a moderate S/N in one ofthe optical states defined by one illumination polarization state and afeeble S/N in another of the optical states defined by a differentillumination polarization state, the defect S/N in the additional imagedata can be increased relative to both optical states because fusing theinformation from the two optical states can both suppress noise andenhance signal. In addition, if a detect produces marginal S/Ns in twooptical states defined by different illumination polarization states,the defect S/N in the additional image data can be increased relative toboth optical states.

Furthermore, if a defect produces appreciable S/Ns in two optical statesdefined by different illumination polarization states and differentcollection polarization states but noise (e.g., from the grain of thewafer) dominates the first and second image data, the noise can besignificantly reduced in the additional image data compared to the firstand second image data by combining the first and second image data asdescribed herein. In a similar manner, if a defect has S/Ns in twooptical states, defined by different illumination polarization statesand different collection polarization states, that are on par with themaximum S/Ns of noise (e.g., from a grain signature) in the first andsecond image data, the noise can be significantly reduced in theadditional image data compared to the first and second image data bycombining the first and second image data as described herein.

In another example, different peak noise events may be present in firstand second image data generated using first and second optical statesdefined by different channels of the inspection system, but a defect mayhave sufficient correlation in the first and second image data such thatby combining the first and second image data as described herein, theS/N of the defect can be dramatically higher in the additional imagedata compared to the first and second image data. In this manner, theembodiments described herein may be used to enhance the detectability ofDOI for wafer inspection systems using information from multiple opticalstates.

An example of how the S/N of a defect can be improved is shown in FIGS.3-5. In particular, output 30 shown in FIG. 3 is a raw image of a bridgedefect from an azimuthal angle of 45 degrees. The S/N (max Diff) of thebridge defect was 1.265 in this image. The S/N was determined using thesignal in signal window 32 and the noise in noise window 34, whichincludes noise from page breaks. Output 36 shown in FIG. 3 is a rawimage of the bridge defect from an azimuthal angle of −45 degrees. TheS/N (max Diff) of the bridge defect was 1.051 in this image. The S/N wasdetermined using the same signal window and noise window as describedabove. In this manner, the raw images of the bridge defect fromazimuthal angles of 45 degrees and −45 degrees show that neither imagehas a sufficient S/N such that the defect can be captured using eitherimage. For example, the S/Ns for the defect are 1.265 and 1.051, whichare both much less than the typical threshold value used for defectdetection.

FIG. 4 is an example of image data for the wafer generated using one ofthe raw images of FIG. 3. For example, image data 38 shown in FIG. 4 isimage data generated by die-to-die subtraction and backgroundsuppression performed using one of the images shown in FIG. 3 and acorresponding reference image from an adjacent die on the wafer. Asshown in FIG. 4, speckle noise appears as many nuisances in this imagedata. In this manner, speckle noise shows as many nuisances even afterdie-to-die subtraction and background suppression. In particular, withthe background reduced, nuisance is still apparent in the image data.More specifically, signals 40 in image data 38 correspond to nuisancefrom speckle on a page break, while signal 42 corresponds to a defect.Therefore, the detectability of the defect will be reduced by thenuisance that remains after die-to-die subtraction and backgroundsuppression.

FIG. 5 is an example of additional image data created by combining theimage data of FIG. 4 with other image data generated for the wafer usingthe other image of FIG. 3. In particular, image data 44 shown in FIG. 5was created by performing image correlation using image data 38 andother image data, image data that was generated from output that wasgenerated using a 45 degree azimuthal angle and image data that wasgenerated from output that was generated using a −45 degree azimuthalangle. After correlation performed using the 45 degree azimuthal angledifference image data and the −45 degree azimuthal angle differenceimage data, the S/N of the defect in the image data created by imagecorrelation is 2. In this manner, the S/N of the defect increases from1.265 and 1.051 to 2. As such, the defect is now detectable with noisegreatly reduced. For example, as shown in FIG. 5, peak noise 46 is theonly speckle peak in the image data, which corresponds to noise that waspresent in both of the difference image data that was correlated. Peaknoise 46 has a gray level of 1044. Second peak noise 48 has a gray levelof 171. In contrast, defect 50 has a gray level of 2060. Therefore, thedefect becomes detectable using the correlated image data. In thismanner, the embodiments have been shown to detect a defect that is notdetectable using either optical state alone (e.g., using image data fromone illumination angle).

As described above, variation of illumination may be used to change thespeckle pattern in image data of the wafer thereby reducing specklenoise after image correlation performed using that image data. Inaddition, although some embodiments are described above as using twoillumination angles that are defined by a 45 degree azimuthal angle anda −45 degree azimuthal angle for the first and second optical states,the different optical states can be extended to various illuminationangles, including changing azimuthal angles and/or elevation angles.Output for each of the different illumination angles may be acquired indifferent passes of the wafer. Correlating image data generated usingmore than two various illumination angles can be used to furthersuppress the noise and improve the S/N. For example, besides changingthe azimuthal angles of illumination, changing elevation angle can alsovary the speckle pattern greatly thereby increasing the un-correlationof noise and further improving the S/N. In this manner, performing themethod using any additional optical state may help to further eliminatethe un-correlated speckle noise and improve the S/N for defects on thewafer. In a similar manner, the correlation can be extended for anychannel and any optical state.

As described above, speckle noise in wafer inspection systems (e.g.,laser-based wafer inspection systems) is one of the main limitations onDOI detection ability. For example, speckle noise increases the noiselevel in inspection image data and reduces S/Ns. Therefore, specklenoise from wafer surface roughness may be one of the main limitations onachievable defect capture rates in some inspection systems. In addition,nuisance detected as a result of wafer noise (e.g., speckle-like noisefrom wafer roughness) is one of the major limitations on DOI detectability. In particular, relatively high nuisance rates and wafer noiseon “rough” wafers such as grainy metal etch wafers may limit theperformance of inspection systems that otherwise have relatively goodoptical resolution. In addition, as wafer design rules continue toshrink, optical inspection systems preferably use shorter wavelengthsand larger collection numerical apertures (NAs). Speckle noiseconsequently becomes a more dominant noise source.

However, as described above, combining the first image data and thesecond image data suppresses speckle noise in the additional image datacreated by the combining step. As such, the methods described herein canbe used to reduce nuisance rates and to improve defect capture rates inwafer inspection systems by reducing a main limiting noise factor,namely speckle noise (e.g., caused by wafer surface roughness).Therefore, the embodiments described herein can be used to increase thesensitivity of wafer inspection systems. In addition, as describedabove, the embodiments described herein allow preservation ofillumination coherence while reducing speckle noise thereby enabling theusage of Fourier filtering and improving the S/N.

In one embodiment, the method includes detecting defects on the waferusing the first image data, detecting defects on the wafer using thesecond image data, and reporting the defects detected on the wafer as acombination of the defects detected using any of the first image data,the second image data, and the additional image data. For example,defects are detected as described above using the additional image data.In a similar manner, defect detection may be separately performed usingthe first image data and the second image data. Defect detectionperformed separately using each of the different image data may beperformed in substantially the same manner (e.g., using the samethreshold value(s)). In this manner, the method may include detectingthree sub-populations of defects (i.e., defects detected using the firstimage data, defects detected using the second image data, and defectsdetected using the additional image data). The three-subpopulations maythen be combined to generate the defect population for the wafer. Forexample, the defect sub-populations may be combined using an OR functionbased on the image data in which the defect was detected. Any defectsthat are detected at substantially the same position in any two or moreof the image data may be reported only once to avoid double reporting ofany one defect. In this manner, any one defect that is detected in twodifferent image data may be reported only once. The defects detected onthe wafer may otherwise be reported in any suitable manner.

As described above, the method may also include generating differentadditional image data. That different additional image data may also beused for defect detection as described above. Any defects detected usingthat different additional image data may be combined with defectsdetected using any other image data (e.g., the additional image data,the first image data, the second image data, etc.) as described herein.Furthermore, if the wafer is scanned using more than two differentoptical states of the inspection system, image data generated using theoutput from the third, fourth, etc. optical states may also be used fordefect detection, and any defects detected using that image data may becombined with defects detected using other image data (e.g., theadditional image data, the first image data, the second image data,etc.) as described herein.

As described above, a defect that is not detectable in either of thefirst and second optical states may become detectable in the additionalimage data created by image correlation. In this manner, the additionalimage data may be used to detect defects on the wafer that are unique inthat they are not or cannot be detected using either the first or secondimage data. As such, the defects detected using the additional imagedata may be used to supplement the inspection results with defects thatwere not or could not be detected by either optical state individually.

The embodiments described herein may also include defect feature levelfusion across multiple optical states, which may provide opportunitiesfor enhancement of separation between DOI and nuisances although bothmay have relatively high S/Ns. For example, in one embodiment, themethod includes determining values for features of the defects using theadditional image data. In this manner, “cross-optical state” features ofdefects can be determined by performing feature calculations based onfused image data. The defect features that are determined using theadditional image data may include any suitable defect features, whichmay be determined in any suitable manner. In this manner, the additionalimage data may be treated as any other image data when it comes todefect feature determination.

In another embodiment, the method includes determining values forfeatures of the defects using some combination of the first image data,the second image data, and the additional image data. In this manner,the “cross-optical state” features can be determined using method(s)and/or algorithm(s) similar to those used to determine “cross-channel”features. For example, defect features may be determined separatelyusing different image data corresponding to the different opticalstates. Those defect features may then be combined to determine adifferent defect feature for the defect. For example, the values fordefect features determined separately using different image datacorresponding to different optical states may be combined into a newvalue for the defect feature. In another example, different defectfeatures may be determined for a defect from different image datacorresponding to different optical states. Those different defectfeatures may then be used in some combination to determine anotherdefect feature for the defect. The manner in which the different imagedata is used to determine the defect features may vary depending on thedefects for which the features are being determined, the features thatare being determined, and the image data itself (e.g., characteristicsof the image data that may affect if or how well a feature can bedetermined using the image data). In this manner, values for features ofthe defects may be determined using all of the information that isavailable or some subset of the information that is available.

Nuisance filtering can be performed not only in the dimension of eachindividual optical state, but in the n-dimensional space generated bythe multiple optical states, which provides more possibilities foridentifying separations between nuisances and DOI. For example, in oneembodiment, detecting the defects includes identifying potential defectson the wafer using the additional image data and identifying the defectsby performing nuisance filtering of the potential defects using pixellevel information about the potential defects determined using the firstimage data, the second image data, the additional image data, or somecombination thereof. Therefore, nuisance filtering can be performed bycombining the information at the pixel level across multiple opticalstates (multiple passes), which creates more potential for performance.The potential defects may be identified in the additional image data bydefect detection, which may be performed as described herein. Nuisancefiltering as described above may also be performed for potential defectsidentified using any other image data described herein.

In another embodiment, detecting the defects includes identifyingpotential defects on the wafer using the additional image data andidentifying the defects by performing nuisance filtering of thepotential defects using values for features of the potential defectsdetermined using the first image data, the second image data, theadditional image data, or some combination thereof. Therefore, nuisancefiltering can be performed by combining information at the feature levelacross multiple optical states (e.g., multiple passes), which createsmore potential for performance. Identifying the potential defects usingthe additional image data may be performed as described above. Thevalues for the features of the potential defects may include any of thevalues of any of the features described herein and may be determined asdescribed herein. Nuisance filtering as described above may also beperformed for potential defects identified using any other image datadescribed herein.

The embodiments described herein may also include image fusion andbinning for wafer inspection systems. Binning can be performed not onlyin the dimension of each individual optical state, but also in then-dimensional space generated by the multiple optical states, whichprovides more possibilities for finding separations between differenttypes of defects. For example, in one embodiment, the method includesbinning the defects using pixel level information about the defectsdetermined using the first image data, the second image data, theadditional image data, or some combination thereof. Therefore, binning,the defects may be performed by combining, information at the pixellevel across multiple optical states (e.g., multiple passes), whichcreates more potential for performance.

In another embodiment, the method includes binning the defects usingvalues for features of the defects determined using the first imagedata, the second image data, the additional image data, or somecombination thereof. Therefore, binning the defects may be performed bycombining information at the defect feature level across multipleoptical states (e.g., multiple passes), which creates more potential forperformance. The values for the features of the defects determined usingthe first image data, the second image data, the additional image data,or some combination thereof may be determined as described furtherherein.

As described above, different additional image data may be generated fora wafer by combining image data corresponding to different combinationsof optical states of an inspection system. In other words, differentfused image data may be generated for a water. The different fused imagedata may be used in all of the steps described herein in the same manneras the additional image data. In addition, if different fused image datais generated for a wafer, that different fused image data may be used todetermine an appropriate or optimal inspection recipe for a wafer. Forexample, the different fused image data may be used independently fordefect detection, which may be performed as described herein. Thedefects that are detected using the different fused image data may thenbe compared. The defects that are uniquely detected using the differentfused image data as compared to the individual optical states may bereviewed by defect review to determine which fused image data detectedthe most unique DOI. That same fused image data may then be created forother wafers of the same process and/or layer and used for detectingdefects on those wafers. In this manner, one or more parameters of aninspection recipe may be determined experimentally using the fused imagedata (i.e., using fused image data generated from experimentallyacquired output for a wafer). In addition, the multiple optical statesused in the methods described herein may be determined in this mannerusing a defect detection method or algorithm (e.g., the defect detectionalgorithm in the inspection recipe). As such, the algorithm may be usedto perform mode selection for an inspection recipe.

Such an approach to determining one or more parameters of an inspectionrecipe may be used to determine the two or more optical states that canadvantageously be used in the methods described herein for apredetermined defect detection method or algorithm. However, such anapproach to determining one or more parameters of an inspection recipemay also be used to determine one or more defect detection parameters(e.g., a defect detection method or algorithm) that should be used withthe two or more optical states. In this manner, the fused image data maybe used to determine any parameter(s) of an inspection recipe.

As described above, the image data that is fused may be generated fromoutput generated using different optical states of a single inspectionsystem. However, the methods described herein are not limited to justfusing image data that is generated from output generated usingdifferent optical states of a single inspection system. For example, inaddition or alternatively, the image data that is fused may includeimage data generated from output acquired in different passes (i.e.,scans) but with the same optical state. In one such embodiment,generating the output as described above using first and second opticalstates of the inspection system is performed in one pass. In thisembodiment, the method also includes generating additional output forthe wafer by scanning the wafer in a different pass with the inspectionsystem using the first or second optical state of the inspection system.In this manner, output using the same (first or second) optical statemay be generated in different passes of the wafer performed by theinspection system. Generating the additional output may be furtherperformed as described herein.

Such a method may also include generating different image data for thewafer using the additional output generated in the different pass.Generating the different image data may be performed as describedfurther herein. In addition, such a method may include combining thedifferent image data with the first image data if the different pass isperformed using the first optical state or the second image data if thedifferent pass is performed using the second optical state correspondingto substantially the same locations on the wafer thereby creatingfurther additional image data for the wafer. In this manner, thecombining step may be performed using the image data acquired indifferent passes but with the same optical state. This combining stepmay be further performed as described herein.

Such a method may further include detecting defects on the wafer usingthe further additional image data. Detecting the defects on the waferusing the further additional image data may be performed as describedfurther herein. Such methods may also include any other step(s)described herein.

Additional methods may include fusing image data, that corresponds todifferent passes of the wafer performed using the same optical state ofan inspection system. For example, another embodiment relates to adifferent method for detecting defects on a wafer. This method includesgenerating output for a wafer by scanning the wafer with an inspectionsystem in first and second passes using a first optical state of theinspection system. Generating the output in this step may be performedas described further herein. The first optical state may include any ofthe optical states described herein.

This method also includes generating first image data for the waferusing the output generated in the first pass and second image data forthe wafer using the output generated in the second pass. Generating thefirst and second image data in this step may be performed as describedfurther herein. The first and second image data may include any of theimage data described herein.

This method further includes combining the first image data and thesecond image data corresponding to substantially the same locations onthe wafer thereby creating additional image data for the wafer.Combining the first image data and the second image data in this stepmay be performed as described further herein. The additional image datama include any of the additional image data described herein. Inaddition, the method includes detecting defects on the wafer using theadditional image data. Detecting the defects on the wafer in this stepmay be performed as described further herein. This method may includeany other step(s) described herein.

Fusing image data from the same optical state but from different passesmay be particularly useful for cases in which the image data isdominated by random noise. For example, if image data is generated usingoutput generated in a first pass using one optical state and fused withimage data generated using output generated in a second pass using thesame optical state, all random noise sources may be substantiallyeliminated in the fused image data, while ensuring coincidence of thesignal from DOI. In addition, as described above, different image datacorresponding to the same optical state can be fused independently. Inother words, first and second image data corresponding to the sameoptical state does not need to be combined with image data correspondingto an already fused optical state (corresponding to a different opticalstate).

Furthermore, in addition or alternatively, the method may be performedusing output generated by different inspection systems. For example, inone embodiment, the method includes generating output for the wafer byscanning the wafer with a different inspection system. Generating theoutput for the wafer with the different inspection system may beperformed as described herein. The different inspection system may be aDF or BF system. For example, the inspection system may be a DF system,and the different inspection system may be a BF system. In anotherexample, the inspection system may be a DF system, and the differentinspection system may be a different DF system (e.g., a DF system havinga different configuration than the inspection system). The differentinspection systems may be configured as described herein or may have anyother suitable configuration known in the art.

Such a method may also include generating third image data for the waferusing the output generated using the different inspection system.Generating the third image data may be performed according to any of theembodiments described herein. In addition, such a method may includecombining the third image data with the first or second image datacorresponding to substantially the same locations on the wafer therebycreating further additional image data for the wafer. Combining thethird image data with the first or second image data may be performed inthis embodiment according to any of the embodiments described herein.Such a method may further include detecting defects on the wafer usingthe further additional image data. Detecting the defects on the waferusing the further additional image data may be performed as describedfurther herein. Such an embodiment may include any other step(s)described herein. In this manner, the method may include using outputcollected from different inspection systems and fusing image datagenerated using the output collected from the different inspectionsystems.

In another embodiment, a method for detecting defects on a waferincludes generating output for a wafer by scanning the wafer with firstand second inspection systems. Generating the output for the wafer inthis step may be performed as described further herein. The first andsecond inspection systems may include any of the different inspectionsystems described herein. Scanning the wafer with the first and secondinspection systems may be performed using the same or substantially thesame optical state. Alternatively, scanning the wafer with the first andsecond inspection systems may be performed using different opticalstates. In this manner, the different optical states corresponding tothe different image data that is fused may be different optical statesof different inspection systems. For example, image data correspondingto optical state A of inspection system X can be combined as describedfurther herein with image data corresponding to optical state B ofinspection system Y. Optical states A and B can be identical ordifferent. The same or substantially the same optical state and thedifferent optical states may include any of the optical states describedherein. The method also includes generating first image data for thewafer using the output generated using the first inspection system andsecond image data for the wafer using the output generated using thesecond inspection system. Generating the first and second image data maybe performed as described further herein. The first and second imagedata may include any of the image data described herein.

The method further includes combining the first image data and thesecond image data corresponding to substantially the same locations onthe wafer thereby creating additional image data for the wafer. Thiscombining step may be performed as described further herein. Inaddition, the method includes detecting defects on the wafer using theadditional image data. Detecting the defects on the wafer in this stepmay be performed as described further herein.

As described above, the method may include generating output usingdifferent inspection systems and generating different image data usingthe output generated using the different inspection systems. However,the method may not necessarily include generating the output using allof the different inspection systems. For example, the output generatedusing one or more of the different inspection systems may be acquiredfrom one or more storage media in which the output has been stored(e.g., by the one or more different inspection systems). The acquiredoutput generated using the different inspection systems may then be usedas described further herein. In this manner, the methods describedherein can perform image fusion regardless of the origin of the outputused to generate the image data that is fused.

In addition, the output generated by the different inspection systemsthat is used in the embodiments described herein may necessarily begenerated using different optical states (as would be the case if thedifferent inspection systems include a DF inspection system and a BFinspection system or DF inspection systems having completely different(e.g., non-overlapping) configurations). However, the output generatedby the different inspection systems that is used in the embodimentsdescribed herein may be generated using the same optical state orsubstantially the same optical state (as may be the case if thedifferent inspection systems include two inspection systems having thesame configuration or relatively similar configurations).

The embodiments described herein may also include storing results of oneor more steps of one or more methods described herein in a storagemedium. The results may include any of the results described herein. Theresults may be stored in any manner known in the art. The storage mediummay include any suitable storage medium known in the art. After theresults have been stored, the results can be accessed in the storagemedium and used by any of the method or system embodiments describedherein, any other method, or any other system. Furthermore, the resultsmay be stored “permanently,” “semi-permanently,” temporarily, or forsome period of time. For example, the storage medium may be randomaccess memory (RAM), and the results may not necessarily persistindefinitely in the storage medium.

Another embodiment relates to a computer-readable medium that includesprogram instructions executable on a computer system for performing acomputer-implemented method for detecting defects on a wafer. One suchembodiment is shown in FIG. 6. For example, as shown in FIG. 6,computer-readable medium 52 includes program instructions 54 executableon computer system 56 for performing a computer-implemented method fordetecting defects on a wafer.

The computer-implemented method includes acquiring output for a wafergenerated by scanning the wafer with an inspection system using firstand second optical states of the inspection system. The first and secondoptical states are defined by different values for at least one opticalparameter of the inspection system. The first and second optical statesmay include any of the optical states described herein. The differentvalues for the at least one optical parameter of the inspection systemmay include any of the different values described herein. The at leastone optical parameter of the inspection system may include any of theoptical parameters described herein. The inspection system may includeany of the inspection systems described herein.

Acquiring the output generated for the wafer may be performed using theinspection system. For example, acquiring the output may include usingthe inspection system to scan light over the wafer and to generateoutput responsive to light scattered from the wafer detected by theinspection system during scanning. In this manner, acquiring the outputmay include scanning the wafer. However, acquiring the output does notnecessarily include scanning the wafer. For example, acquiring theoutput may include acquiring the output from a storage medium in whichthe output has been stored (e.g., by the inspection system). Acquiringthe output from the storage medium may be performed in any suitablemanner, and the storage medium from which the output is acquired mayinclude any of the storage media described herein. The output mayinclude any of the output described herein.

The computer-implemented method also includes generating first imagedata for the wafer using the output generated using the first opticalstate and second image data for the wafer using the output generatedusing the second optical state. Generating the first image data and thesecond image data may be performed as described further herein. Thefirst and second image data may include any such image data describedherein.

The computer-implemented method further includes combining the firstimage data and the second image data corresponding to substantially thesame locations on the wafer thereby creating additional image data forthe wafer. Combining the first image data and the second image data maybe performed as described further herein. The additional image data mayinclude any of the additional image data described herein. In addition,the method includes detecting defects on the wafer using the additionalimage data. Detecting the defects on the wafer may be performed asdescribed further herein. The defects detected on the wafer may includeany of the defects described herein. The computer-implemented method forwhich the program instructions are executable may include any otherstep(s) of any other method(s) described herein.

Program instructions 54 implementing methods such as those describedherein may be transmitted over or stored on computer-readable medium 52.The computer-readable medium may be a storage medium such as a read-onlymemory, a RAM, a magnetic or optical disk, or a magnetic tape or anyother suitable computer-readable medium known in the art.

The program instructions may be implemented in any of various ways,including procedure-based techniques, component-based techniques, and/orobject-oriented techniques, among others. For example, the programinstructions may be implemented using Matlab, Visual Basic, ActiveXcontrols, C, C++ objects, C#, JavaBeans, Microsoft Foundation Classes(“MFC”), or other technologies or methodologies, as desired.

Computer system 56 may take various forms, including a personal computersystem, mainframe computer system, workstation, system computer, imagecomputer, programmable image computer, parallel processor, or any otherdevice known in the art. In general, the term “computer system” may bebroadly defined to encompass any device having one or more processors,which executes instructions from a memory medium.

An additional embodiment relates to a system configured to detectdefects on a wafer. One embodiment of such a system is shown in FIG. 7.As shown in FIG. 7, system 58 includes inspection subsystem 60 andcomputer subsystem 80. The inspection subsystem is configured togenerate output for a wafer by scanning the wafer using first and secondoptical states of the inspection subsystem. For example, as shown inFIG. 7, the inspection subsystem includes light source 62. Light source62 may include any suitable light source known in the art such as alaser. Light source 62 is configured to direct light to polarizingcomponent 64, which may include any suitable polarizing component knownin the art. In addition, the inspection subsystem may include more thanone polarizing component (not shown), each of which may be positionedindependently in the path of the light from the light source. Each ofthe polarizing components may be configured to alter the polarization ofthe light from the light source in a different manner. The inspectionsubsystem may be configured to move the polarizing components into andout of the path of the light from the light source in any suitablemanner depending on which polarization setting is selected forillumination of the wafer during a scan. The polarization setting usedfor the illumination of the wafer during a scan may include P, S, orcircularly polarized (C).

Light exiting polarizing component 64 is directed to wafer 66 at anoblique angle of incidence, which may include any suitable oblique angleof incidence. The inspection subsystem may also include one or moreoptical components (not shown) that are configured to direct light fromlight source 62 to polarizing component 64 or from polarizing component64 to wafer 66. The optical components may include any suitable opticalcomponents known in the art such as, but not limited to, a reflectiveoptical component. In addition, the light source, the polarizingcomponent, and/or the one or more optical components may be configuredto direct the light to the wafer at one or more angles of incidence(e.g., an oblique angle of incidence and/or a substantially normal angleof incidence). The inspection subsystem may be configured to perform thescanning by scanning the light over the wafer in any suitable manner.

Light scattered from wafer 66 may be collected and detected by multiplechannels of the inspection subsystem during scanning. For example, lightscattered from wafer 66 at angles relatively close to normal may becollected by lens 68. Lens 68 may include a refractive optical elementas shown in FIG. 7. In addition, lens 68 may include one or morerefractive optical elements and/or one or more reflective opticalelements. Light collected by lens 68 may be directed to polarizingcomponent 70, which may include any suitable polarizing component knownin the art. In addition, the inspection subsystem may include more thanone polarizing component (not shown), each of which may be positionedindependently in the path of the light collected by the lens. Each ofthe polarizing components may be configured to alter the polarization ofthe light collected by the lens in a different manner. The inspectionsubsystem may be configured to move the polarizing components into andout of the path of the light collected by the lens in any suitablemanner depending on which polarization setting is selected for detectionof the light collected by lens 68 during scanning. The polarizationsetting used for the detection of the light collected by lens 68 duringscanning may include any of the polarization settings described herein(e.g., P, S, and N).

Light exiting polarizing component 70 is directed to detector 72.Detector 72 may include any suitable detector known in the art such as acharge coupled device (CCD) or another type of imaging detector.Detector 72 is configured to generate output that is responsive to thescattered light collected by lens 68 and transmitted by polarizingcomponent 70 if positioned in the path of the collected scattered light.Therefore, lens 68, polarizing component 70 if positioned in the path ofthe light collected by lens 68, and detector 72 form one channel of theinspection subsystem. This channel of the inspection subsystem mayinclude any other suitable optical components (not shown) known in theart such as a Fourier filtering component.

Light scattered from wafer 66 at different angles may be collected bylens 74. Lens 74 may be configured as described above. Light collectedby lens 74 may be directed to polarizing component 76, which may includeany suitable polarizing component known in the art. In addition, theinspection subsystem may include more than one polarizing component (notshown), each of which may be positioned independently in the path of thelight collected by the lens. Each of the polarizing components may beconfigured to alter the polarization of the light collected by the lensin a different manner. The inspection subsystem may be configured tomove the polarizing components into and out of the path of the lightcollected by the lens in any suitable manner depending on whichpolarization setting is selected for detection of the light collected bylens 74 during scanning. The polarization setting used for detection ofthe light collected by lens 74 during scanning may include P, S, or N.

Light exiting polarizing component 76 is directed to detector 78, whichmay be configured as described above. Detector 78 is also configured togenerate output that is responsive to the collected scattered light thatpasses through polarizing component 76 if positioned in the path of thescattered light. Therefore, lens 74, polarizing component 76 ifpositioned in the path of the light collected by lens 74, and detector78 may form another channel of the inspection subsystem. This channelmay also include any other optical components (not shown) describedabove. In some embodiments, lens 74 may be configured to collect lightscattered from the wafer at polar angles from about 20 degrees to about70 degrees. In addition, lens 74 may be configured as a reflectiveoptical component (not shown) that is configured to collect lightscattered from the wafer at azimuthal angles of about 360 degrees.

The inspection subsystem shown in FIG. 7 may also include one or moreother channels (not shown). For example, the inspection subsystem mayinclude an additional channel, which may include any of the opticalcomponents described herein such as a lens, one or more polarizingcomponents, and a detector, configured as a side channel. The lens, theone or more polarizing components, and the detector may be furtherconfigured as described herein. In one such example, the side channelmay be configured to collect and detect light that is scattered out ofthe plane of incidence (e.g., the side channel may include a lens, whichis centered in a plane that is substantially perpendicular to the planeof incidence, and a detector configured to detect light collected by thelens).

The first and second optical states are defined by different values forat least one optical parameter of the inspection subsystem. The firstand second optical states may be defined by any of the different valuesfor any of the optical parameters of the inspection subsystem describedherein. In addition, the values of any of the optical parameters may bealtered in any suitable manner if necessary between passes. For example,if the different values are different values of illuminationpolarization states, between passes polarizing component 64 may beremoved and/or replaced as described herein with a different polarizingcomponent. In another example, if the different values are differentangles, the position of the light source and/or any other opticalcomponents (e.g., polarizing component 64) used to direct the light tothe wafer may be altered between passes in any suitable manner.

Output generated by the detectors during scanning may be provided tocomputer subsystem 80. For example, the computer subsystem may becoupled to each of the detectors (e.g., by one or more transmissionmedia shown by the dashed lines in FIG. 7, which may include anysuitable transmission media known in the art) such that the computersubsystem may receive the output generated by the detectors. Thecomputer subsystem may be coupled to each of the detectors in anysuitable manner. The output generated by the detectors during scanningof the wafer may include any of the output described herein.

The computer subsystem is configured to generate first image data forthe wafer using the output generated using the first optical state andsecond image data for the wafer using the output generated using thesecond optical state. The computer subsystem may be configured togenerate the first image data and the second image data according to anyof the embodiments described herein. The first image data and the secondimage data may include any such image data described herein.

The computer subsystem is also configured to combine the first imagedata and the second image data corresponding to substantially the samelocations on the wafer thereby creating additional image data for thewafer. The computer subsystem may be configured to combine the first andsecond image data according to any of the embodiments described herein.The additional image data may include any of the additional image datadescribed herein.

The computer subsystem is further configured to detect defects on thewafer using the additional image data. The computer subsystem may beconfigured to detect the defects according to any of the embodimentsdescribed herein. The defects may include any of the defects describedherein.

The computer subsystem may be configured to perform any other step(s) ofany method embodiment(s) described herein. The computer subsystem may befurther configured as described herein. The inspection subsystem mayalso be further configured as described herein. Furthermore, the systemmay be further configured as described herein.

It is noted that FIG. 7 is provided herein to generally illustrate oneconfiguration of an inspection subsystem that may be included in thesystem embodiments described herein. Obviously, the inspection subsystemconfiguration described herein may be altered to optimize theperformance of the inspection subsystem as is normally performed whendesigning a commercial inspection system. In addition, the systemsdescribed herein may be implemented using an existing inspection systemby adding functionality described herein to an existing inspectionsystem) such as the Puma 9000 and 91xx series of tools that arecommercially available from KLA-Tencor. For some such systems, themethods described herein may be provided as optional functionality ofthe system (e.g., in addition to other functionality of the system).Alternatively, the system described herein may be designed “fromscratch” to provide a completely new system.

Further modifications and alternative embodiments of various aspects ofthe invention may be apparent to those skilled in the art in view ofthis description. For example, systems and methods for detecting defectson a wafer are provided. Accordingly, this description is to beconstrued as illustrative only and is for the purpose of teaching thoseskilled in the art the general manner of carrying out the invention. Itis to be understood that the forms of the invention shown and describedherein are to be taken as the presently preferred embodiments. Elementsand materials may be substituted for those illustrated and describedherein, parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

What is claimed is:
 1. A method for detecting defects on a wafer,comprising: generating output for a wafer by scanning the wafer with aninspection system in first and second passes using a first optical stateof the inspection system; generating first image data for the waferusing the output generated in the first pass and second image data forthe wafer using the output generated in the second pass; combining thefirst image data and the second image data corresponding tosubstantially the same locations on the wafer thereby creatingadditional image data for the wafer; detecting defects on the waferusing the additional image data; and generating output for the wafer byscanning the wafer with a different inspection system, generating thirdimage data for the wafer using the output generated using the differentinspection system, combining the third image data with the first orsecond image data corresponding to substantially the same locations onthe wafer thereby creating further additional image data for the wafer,and detecting defects on the wafer using the further additional imagedata.
 2. The method of claim 1, wherein scanning the wafer with theinspection system using the first optical state is performed withcoherent light.
 3. The method of claim 1, further comprising generatingadditional output for the wafer by scanning the wafer in a differentpass with the inspection system using a second optical state of theinspection system, generating different image data for the wafer usingthe additional output generated in the different pass, combining thedifferent image data with the first or second image data correspondingto substantially the same locations on the wafer thereby creating otheradditional image data for the wafer, and detecting defects on the waferusing the other additional image data.
 4. The method of claim 1, whereinthe first and second image data comprises difference image data.
 5. Themethod of claim 1, wherein said combining comprises performing imagecorrelation on the first image data and the second image datacorresponding to substantially the same locations on the wafer.
 6. Themethod of claim 1, wherein said combining is performed at the pixellevel of the first and second image data.
 7. The method of claim 1 ,wherein defect detection is not performed prior to the combining step.8. The method of claim 1, wherein portions of the additional image datathat correspond to the defects have greater signal-to-noise ratios thanportions of the first and second image data that are combined to createthe portions of the additional image data.
 9. The method of claim 1,wherein the additional image data has less noise than the first andsecond image data.
 10. The method of claim 1, wherein the additionalimage data has less speckle noise than the first and second image data.11. The method of claim 1, further comprising detecting defects on thewafer using the first image data, detecting defects on the wafer usingthe second image data, and reporting the defects detected on the waferas a combination of the defects detected using any of the first imagedata, the second image data, and the additional image data.
 12. Themethod of claim 1, further comprising determining values for features ofthe defects using the additional image data.
 13. The method of claim 1,further comprising determining values for features of the defects usingsome combination of the first image data, the second image data, and theadditional image data.
 14. The method of claim 1, wherein detecting thedefects comprises identifying potential defects on the wafer using theadditional image data and identifying the defects by performing nuisancefiltering of the potential defects using pixel level information aboutthe potential defects determined using the first image data, the secondimage data, the additional image data, or some combination thereof. 15.The method of claim 1, wherein detecting the defects comprisesidentifying potential defects on the wafer using the additional imagedata and identifying the defects by performing nuisance filtering of thepotential defects using values for features of the potential defectsdetermined using the first image data, the second image data, theadditional image data, or some combination thereof.
 16. The method ofclaim 1, further comprising binning the defects using pixel levelinformation about the defects determined using the first image data, thesecond image data, the additional image data, or some combinationthereof.
 17. The method of claim 1, further comprising binning thedefects using values for features of the defects determined using thefirst image data, the second image data, the additional image data, orsome combination thereof.